A fireball is seen across Interstate 77 in Sissonville, West Virginia in this aerial photo from December 11. A natural gas pipeline exploded in flames near Charleston, West Virginia, on Tuesday, setting nearby buildings on fire and injuring several people, authorities said.

The fireball explosion Tuesday of an interstate natural gas transmission line in West Virginia, which left behind a huge jet of flame that burned for more an hour and melted four lanes of I-77, is just one of scores of accidents and explosions involving natural gas lines this year, federal data show.

Despite the magnitude of the explosion and fire, preliminary reports were that all persons were accounted for with no injuries, said a member of the National Transportation Safety Board, which is charged by Congress with investigating pipeline as well as airline, railroad and other transportation accidents.

An NTSB team was in Sissonville, W.V. at first light Wednesday, examining evidence at the accident site.

Robert Sumwalt, a safety board member, told reporters at an initial press briefing late Tuesday that the NTSB team would not speculate on causes of the explosion, but would collect evidence and interview witnesses, including the operators of the pipeline, Columbia Gas Transmission company, a subsidiary of Houston-based NiSource Gas Transmission and Storage.

But whatever cause eventually emerges, the dramatic event in Sissonville is set against a backdrop of several high profile natural gas accidents and a rapid increase nationally in gas pipeline mileage – even as federal oversight appears to lag.

"There are never enough inspectors at the state or federal level to adequately cover all the pipelines," says Rebecca Craven, program director at the Pipeline Safety Trust, a watchdog group based in Bellingham, Wash., that monitors energy pipelines of all types. "They can't physically spend enough time with each operator or pipeline to be able to do a thorough job and conduct regular inspections. They do what they can ­– enough to comply with their requirements."

The explosion at Sissonville adds to a previous tally of 80 small and large incidents this year involving just natural gas transmission lines, the big pipelines that ship huge quantities of gas from production areas to distribution hubs and population centers across the country, according to the Pipeline Hazardous Materials Safety Administration (PHMSA), a branch of the US Department of Transportation that inspects and regulates the nation's pipelines.

Of the 80 incidents, 38 were classified as significant, PHMSA data show. The accidents and fires reportedly caused seven injuries, no fatalities, and $44 million of damage.

Added to this year's accident tally for gas transmission lines were another 71 incidents with nine fatalities and 21 injuries involving natural gas distribution lines, the much smaller gas lines under lower pressure that bring gas directly to residential and commercial customers in and around major population centers, the PHMSA data show.

The nation's energy transportation network includes more than 2.5 million miles of pipeline operated by about 3,000 companies of all sizes. That includes 321,000 miles of onshore and offshore “gas transmission and gathering” pipelines and another 2 million miles of gas distribution pipelines. Yet PHMSA has funding for only 137 inspectors, and often employs even less than that (in 2010 the agency had 110 inspectors on staff), ProPublica reported last month.

At the same time, a Congressional Research Service report found that staffing shortfalls have for many years averaged 24 employees a year. And a New York Times investigation last year concluded that PHMSA simply lacks funding to hire more inspectors. The Obama administration's budget requests additional funding to hire more inspectors.

What those inspections need to catch, Ms. Craven says, is a variety of problems ranging from improper welds, to faulty machinery, to improper operational procedures. In the past year, 41 percent of natural gas transmission pipeline failures were due to materials, welding, and faulty equipment, with corrosion causing 10 percent of the accidents, PHMSA data show.

A few facts in the Sissonville case are known, Sumwalt said. The explosion, for instance, burned more than an hour, from 12:41 p.m. Tuesday until 1:46 p.m. Why it took the company an hour and four minutes to shut down the 30-inch pipeline down is something likely to be examined by investigators. It was operating at 929 pounds per square inch of pressure, below its maximum of 1,000 p.s.i., he said.

In the infamous 2010 natural gas pipeline explosion and fire in San Bruno, Calif., that killed eight people and destroyed 38 homes, it took about 95 minutes for pipeline owner Pacific Gas & Electric Co. to shut off the gas line. Lack of automatic shut-off valves and of valves that can be remotely shut was partly to blame for the sluggish response, NTSB investigators concluded in their report.

Federal regulators have cited 20 significant incidents involving deaths, injuries, or major property damage in West Virginia in the last decade involving gas explosions, the Associated Press reported. Congress may also be peering into the matter.

For its part, Columbia Gas Transmission transports an average of 3 billion cubic feet of natural gas per day through a nearly 12,000-mile pipeline network and 92 compressor stations in 10 states, serving hundreds of communities, according to the company's website. Customers include local gas distribution companies, energy marketers, electric power generating facilities, as well as industry.

Federal data show that only a little more than one-third of Columbia Gas is federally inspected with the rest subject to state inspection. In the past year, there were 34 accidents on the company's lines resulting in five injuries and $12.3 million in damage, PHSMA data shows. At this point, however, it's impossible to know just what caused the Sissonville accident. Company officials expressed gratitude that so far there have been no reported injuries. Workers restored the highway to passable condition overnight.

Sancha Adkins, from St. Albans, W.Va., told the Associated Press that she was headed north on I-77 when a flash alongside the highway caught her eye. She slammed on the brakes and pulled to the shoulder, as did the tractor-trailer behind her, just in time to see a wall of flame roar across the road about 150 feet ahead of her.

She tried to back up, but the truck behind her wasn't doing the same fast enough.

"I did a U-turn in the middle of the road and literally drove the wrong way on the interstate. I had my hazard lights on flashing, just trying to tell people to get out of the way," she said. There was oncoming traffic as she hugged the berm on the median.